This invention pertains to a method and apparatus for modifying a video signal such that a television receiver may still produce a normal picture from the modified signal, while a videotape recording of the modified signal results in generally unacceptable pictures.
To protect valuable rights in video information, there exists a need for a method and apparatus for modifying a video signal so that a normal color picture may be produced by a television receiver receiving the modified video signal, but videotape recording of the modified video signal is inhibited or prevented. In a copending application by the present inventor and assigned to the assignee of the present invention, application Ser. No. 554,697, filed in the United States Patent and Trademark Office on Nov. 23, 1983, there is disclosed and described a modified video signal directed to the desirable object stated above. In other words, a television set produces a normal color picture from the modified video signal of Ser. No. 554,697, while a videocassette recording reproduces unacceptable pictures.
The invention there disclosed depends on the fact that if a pulse of suitable amplitude and duration is added to the video signal immediately following the trailing edge of a sync pulse, then an automatic gain control (AGC) system in the video-cassette recorder will behave as though the incoming video signal level is about 350% of normal level. The automatic gain control system responds by reducing the signal recording on the tape to about 30% of its normal value. Because of this reduced value, during replay, this signal will generate poor quality and unstable pictures on a normal television set.
The added pulses of the copending Ser. No. 554,697 application are in a back porch region of the video signal. Black level signal depression may be significant, however, if the pulses are added to the back porch segment on all lines of a picture frame. Therefore, the pulses are actually added only to some percentage of the back porch portions, such as every third line, to reduce significance of the black level signal depression problem. The disclosure of the Ser. No. 554,697 application is herein incorporated by reference.
The present inventor has now discovered a new invention for "confusing" or causing misoperation of the AGC system in a videocassette recorder while solving the black level depression problem. The present invention includes both method and apparatus for modifying a video signal so as to permit normal television receiver reproduction thereof while inhibiting acceptable videocassette recording of the modified video signal. In other words, when applied to the program material of a prerecorded videocassette, this invention permits the video recorder to playback clearly the cassette while not permitting the signal therefrom to be successfully recorded with another recorder.